The present invention relates generally to the detection of rotating stall in a centrifugal compressor. More specifically, the present invention relates to systems and methods of detecting rotating stall in the diffuser portion of a centrifugal compressor by sensing acoustic energy changes in the discharge from the compressor.
Rotating stall in a centrifugal compressor can occur in the rotating impeller or rotor of the compressor or in the stationary diffuser of the compressor downstream from the impeller. The frequencies of the energy associated with rotating stall are typically within a common range of values whether the rotating stall is occurring in the impeller region (impeller rotating stall) or in the diffuser region (diffuser rotating stall). In both cases, the presence of rotating stall can adversely affect performance of the compressor and/or system. However, impeller rotating stall is typically of greater interest because it can affect impeller reliability, especially in axial flow compressors such as aircraft engines, while diffuser rotating stall typically impacts the overall sound and vibration levels of a system.
Some techniques for detecting and correcting impeller rotating stall use a plurality of sensors circumferentially positioned adjacent to the rotating impeller. The sensors are used to detect disturbances at individual locations. The disturbances are then compared to values at other locations or values corresponding to optimal operating conditions. Often, very complicated computations are performed to determine precursors to the onset of impeller rotating stall. Once impeller rotating stall is detected, some corrective actions include bleeding discharge gas back to the suction inlet of the compressor or modifying suction inlet flow angles using baffles or varying the position of the vanes.
One example of a technique for detecting impeller rotating stall in an axial flow compressor is in U.S. Pat. No. 6,010,303 (the '303 patent). The '303 patent is directed to the prediction of aerodynamic and aeromechanical instabilities in turbofan engines. An instability precursor signal is generated in real-time to predict engine surge, stall or blade flutter in aeropropulsion compression systems for turbofan engines which utilize multistage axial flow compressors. Energy waves associated with aerodynamic or aeromechanical resonances in a compression system for a turbofan engine are detected and a signal indicative of the frequencies of resonance is generated. Static pressure transducers or strain gauges are mounted near or on the fan blades to detect the energy of the system. The real-time signal is band pass filtered within a predetermined range of frequencies associated with an instability of interest, e.g. 250-310 Hz. The band pass signal is then squared in magnitude. The squared signal is then low pass filtered to form an energy instability precursor signal. The low pass filter provides an average of the sum of the squares of each frequency. The precursor signal is then used to predict and prevent aerodynamic and aeromechanical instability from occurring in a turbofan engine. One drawback of this technique is that it is only for the detection of impeller rotating stall in an axial flow compressor and does not discuss diffuser rotating stall.
Mixed flow centrifugal compressors with vaneless radial diffusers can experience diffuser rotating stall during some part, or in some cases, all of their intended operating range. Typically, diffuser rotating stall occurs because the design of the diffuser is unable to accommodate all flows without some of the flow experiencing separation in the diffuser passageway. Diffuser rotating stall results in the creation of low frequency sound energy or pulsations in the gas flow passages at fundamental frequencies that are generally less than the rotating frequency of the compressor's impeller. This low frequency sound energy and its associated harmonics propagate downstream through the compressor gas passageways into pipes, heat exchangers and other vessels. The low frequency sound energy or acoustic disturbances can have high magnitudes and are undesirable because the presence of acoustic disturbances may result in the premature failure of the compressor, its controls, or other associated parts/systems.
Therefore, what is needed is a system and method for detecting and correcting rotating stall in the diffuser of a centrifugal compressor by sensing a change in the acoustic energy in the gas stream around the diffuser and then taking action to modify the compression process to avoid or remedy those conditions that produce significant amounts of rotating stall noise in the diffuser.